Launch the Terminal and type the following command to list volumes. Note, you can select MS-DOS (FAT) for the Format.
Connect your USB to mac computer, then perform Format or Erase for your USB drive by using Disk Utility (under Application/Utilities). Back up your USB drive first as your USB drive will be erased completely before you start.
The first way I recommend is by using dd commend-line under Utilities, which is able to make a byte-for-byte copy of ISO image and partition table, you should know dd is a command line-only interface, which is somewhat advanced and might not suitable for the low skilled mac users. Burn ISO File to USB on Mac in Virtualbox Burn Windows ISO to USB on Mac with Terminal